This invention relates to a rod antenna. More particularly, it concerns a rod antenna provided with a ferrite material along the longitudinal extent thereof, the ferrite material being arranged so as to define the electrical length of the rod antenna upon increasing frequency. The rod antenna is particularly well suited for use together with radio material of the kind that changes its transmission frequency relatively often.
Particularly in context of military radio communication, it is common to use radio material that keeps changing its transmission frequency during communication. The aim is to make undesired tapping of a communication circuit difficult.
Whilst the frequency range of the radio material previously could be, for example, in the range of 30 to 88 MHz, it is now common to operate within a frequency range typically being 30 to 512 MHz. To a person skilled in the art, it is obvious that radio communication across such a wide frequency range places great demands on the antenna, insofar as the antenna must be able to operate with satisfactory electrical properties, such as gain and SWR (Standing Wave Ratio), within the entire frequency range without requiring calibration of the antenna during operation.
The terms gain and SWR will be known to a skilled person and will therefore not be defined further.
In mobile communication, a rod antenna connected to, for example, a vehicle is oftentimes used, and in which the electrical length of the rod antenna may be ¼ of the wavelength. The vehicle may form the ground-plane of the antenna. The antenna in question is a so-called end-fed antenna, in which the rod antenna forms one half of a so-called “Hertzian dipole antenna”.
An antenna of this kind, in which the antenna has a fixed electrical length, is relatively unsuitable for use together with radio material of the type mentioned. The reason for this is that the antenna is electrically resonant at several fixed frequencies, and that the SWR will increase to unacceptable values when the transmission frequency deviates substantially from the resonant frequency.
According to the prior art, this unfavourable condition may be overcome to a certain degree by reducing the electrical length of the antenna as the transmission frequency increases.
Thus, U.S. Pat. No. 6,429,821 discloses an end-fed rod antenna provided with induction coil resistance networks along the length thereof. In principal, these networks are arranged so as to reduce the electrical length of the antenna upon increased frequency by allowing each induction coil resistance network to disconnect, at a corresponding frequency, the part of the antenna located directly opposite the induction coil resistance network in question. Thereby, the electrical length of the rod antenna is continuously adapted to the frequency supplied. Still, the antenna exhibits a considerable variation in gain and SWR across the frequency range of interest.